Analogue modelling (geology)

[1][3] Secondly, the length scales of geodynamic processes are enormous (thousands of kilometres), and most of them happen at depth within the Earth.

[1][3] Thus, scientists began making proportional small-scale simulations of features in the natural world to test geological ideas.

They are helpful in understanding the internal structures and the progressive development of Earth's deforming regions.

[1] Analogue modelling has been widely used for geodynamic analysis and to illustrate the development of different geological phenomena.

[1] It has been used since at least 1812, when James Hall squeezed layers of clay to produce folds similar to those that he had studied at an outcrop.

[1] King Hubbert came up with the theory of scaling in 1937, meaning that the study of analogue modelling became quantitative.

[1] As the field of geodynamic study expanded, analogue modelling increased, especially for large-scale geological processes.

Examples include proto-subduction[9] subduction[10][11] in plate tectonics, collision,[12] diapirism,[13] and rifting.

He defined three types of similarity between models and the natural world: geometric, kinematic and dynamic.

[8][15] To be geometrically similar, lengths in the model and natural example must be proportional and angles must be equal.

(Generating from Cauchy momentum equation[16]) Stokes' law is usually used for showing the relationship between forces and density contrasts (

For example, lateral compression machines are commonly used in simulating deformations involving lithospheric shortening, such as folding,[2] thrust faulting, collision, and subduction.

[18] There is a large variety of devices based on the different sources of forces applied to the material.

[1] For experimental systems, the energy can be supplied externally (at the boundary) and internally (buoyancy forces).

These technologies are usually used in studying development of gravity-controlled structures, such as dome formation,[21] and diapirism.

[1] Analogue modelling uses various materials, such as sand, clay, silicone and paraffin wax.

[22] Before Hubbert's scaling theory, scientists used natural materials (e.g. clays, soil, and sand) for analogue modelling.

[1] For large-scale simulation, analogue modelling should have geometric, kinematic, and dynamic similarity with nature.

[8] All these different materials represent the natural features of Earth (such as crust, mantle, and river).

[22] Selection of analogue materials is difficult, because of the largely rheology-dependent deformation and inconstant rheology influenced by the thermal gradient in nature.

The rheological characteristic of internal layering was developed by the study of seismology and geochemistry.

[1] To simulate layers with different properties, different materials are chosen: Sub-lithospheric mantle There are many useful properties of analogue modelling: Because analogue modelling involves the simplification of geodynamic processes, it also has several disadvantages and limitations:[15] Analogue modelling can be used to simulate different geodynamic processes and geological phenomena, such as small-scale problems – folding, fracturing, boudinage and shear zone, and large-scale problems – subduction, collision, diapirism, and mantle convection.

The first analogue model was built by James Hall for simulating folds.

The use of more complex compression machines substantially increases the number of simulations of compressional tectonics, including subduction, collision, lithospheric shortening, fracture formation, thrust and accretionary wedge.

If the simulation only focuses on the upper crustal, the model is always built in the glass box (or two lateral glass walls) with a piston and/or wedges to supply forces to layers of granular materials (normally called sandbox).

Depending on the different natural features, erosion (removal of top materials at a certain angle), décollement (inserted layers with low cohesion, normally glass microbeads), and any other parameters can put into the model, producing various results.

Because of the different physical and chemical properties between the asthenosphere and lithosphere, viscous materials and a heater (for mantle convection) are also used.

[2] Compression machines can also be used in reverse for simulating extensional tectonics, such as lithospheric extension, the formation of rifts, normal faulting, boudinage and diapirs.

This kind of horizontal movement will create a shear zone and several types of fractures and faults.

Scientists used CT-analysis to collect the cross-section images for the observation of the most influenced area during the simulation.

Pure shear sandbox model of thrust fault formation
Lateral compression machine made by James Hall to model geological folding. This machine is still present in the Royal Society of Edinburgh . The materials squeezed in the box are blankets or layers of clay. [ 2 ]
Analogue model of caldera formation using flour to represent the upper part of the crust and a balloon to represent the inflating magma chamber
The simple analogue modelling of the fold-thrust belt system taken in the Nanjing University physical laboratory. The white and blue materials are quartz sands.
The simple analogue modelling of a subduction zone . The materials this model uses are sand mixture and silicone putty for the continental crust (left in layered brown) and oceanic crust (right in layered brown), and glucose syrup for the asthenosphere (greenish-blue liquid in the glass tank). There is a heater in the tank for heating the liquid. [ 2 ] [ 19 ] [ 20 ]
Simple analogue modelling of the growth and erosion of an orogenic wedge. This simulation is done in a glass tank, with layered different granular materials that represent the crust. [1]
The simple analogue modelling of the extension tectonics which showing the formation of normal fault and salt dome (diapirism). This model is built in a glass box. The darker greyish layer is silicone which represents salt, and brownish layers are dry quartz sands which represent the brittle sedimentary rocks. [ 13 ] [2]
The simplified analogue modelling setting of shear deformation. This model is built on two separate horizontal plates. The brownish layers are dry sand, wet clay, and viscous materials, such as silicone or polydimethylsiloxane. [ 26 ]